The present invention relates to an arc welding apparatus that can selectively switch between output characteristics having two types of high and low no-load voltages by one arc welding apparatus, and in particular, to an arc welding apparatus having a construction for selectively switching between CO.sub.2 welding, MIG (Metal Inert Gas arc) welding or TIG (Tungsten Inert Gas arc) welding that permit a low no-load voltage and manual welding that requires a high no-load voltage.
A prior art arc welding apparatus will be described with reference to FIG. 2.
In FIG. 2 are shown a welding transformer 1, a first thyristor 2 for controlling an output current, a second thyristor 3, a welding method changeover switch S1 for selecting either CO.sub.2 welding or manual welding, a control circuit 6 for controlling the phases of the first and second thyristors, a reactor 7 for smoothing a welding current, a manual welding holder 8 for holding an electrode rod (electrification section) 9 for manual welding, a CO.sub.2 welding torch 10 a base metal (workpiece) 11 for welding.
In performing CO.sub.2 arc welding by this prior art, the anodes of the first thyristor 2 and the second thyristor 3 have been connected by the welding method changeover switch S1 to taps 1a and 1c provided on the secondary winding side of the welding transformer 1, the cathodes of the first thyristor 2 and the second thyristor 3 have been connected to the CO.sub.2 welding torch 10 by the welding method changeover switch S1, and the neutral point of the welding transformer 1 have been connected to the workpiece 11 via the reactor 7 for smoothing the welding current.
In performing manual welding, the anodes of the first thyristor 2 and the second thyristor 3 have been connected to taps 1b and 1d provided on the high voltage side of the welding transformer 1 by the welding method changeover switch S1, and the cathodes of the first thyristor 2 and the second thyristor 3 have been connected to the electrification section 9 of the manual welding holder 8 by the welding method changeover switch S1.
In general, the required output characteristics differ depending on each welding method. The CO.sub.2 arc welding requires an output characteristic of a low voltage and a large current supply capacity, while the manual welding requires a higher voltage than that of the CO.sub.2 arc welding but requires not so large current supply capacity. According to the prior art arc welding apparatus, in order to obtain different output characteristics depending on each welding method by means of one welding apparatus, one pair of the taps 1a, 1b, 1c and 1d provided on the secondary winding of the welding transformer 1 is connected to the anode of the first thyristor 2 or the second thyristor 3 by means of the triple-pole double-throw welding method changeover switch S1.
Then, the large-size triple-pole double-throw welding method changeover switch S1 has been changed over to determine whether to connect the output of the welding apparatus to the CO.sub.2 welding torch or to the electrification section of the manual welding holder, connecting the cathodes of the first and second thyristors 2 and 3 to either the electrification section of the manual welding holder 8 or to the CO.sub.2 welding torch 10.
However, the prior art arc welding apparatus has been required to employ the large-size triple-pole double-throw welding method changeover switch for performing switching of a large current, and this large-size triple-pole double-throw welding method changeover switch has occupied a large part of the material cost of the arc welding apparatus. The contact points of the switch, which turn on and off a large current, tends to wear out soon, and this has led to a disadvantage in terms of contact point operating life.
Furthermore, this large-size triple-pole double-throw welding method changeover switch has been required to be operated until the contact points are mechanically completely turned on.